Stack Vent Leak solutions

My mom discovered open sky when she went up into her attic. She describes a gap around a standing pipe that exits through the roof - like a hole was cut too large for the pipe itself. She wonders if she could use this foaming insulation stuff from under the roof (attic access) that would make a plastic-like seal around the pipe to stop leaking. I’m wondering if she is missing a pipe boot and whether it is an easy fix if I climbed onto the roof. Are these pipes a standard circumference? Thank you.

Don’t do the foam thing.

The most common vent pipe size in my area is 3", some are 1 1/2", 2", 2 1/2", or 4".
The rubber seal on the existing boot may be deteriorated.

If this is the case one of these is a quick, inexpensive, easy fix.

It is called “CapMaster”, made by Mid-America.

If you cannot find this product a local roofer will replace your pipe boot for you, for a nominal fee.

There is one other way to fix this that is relatively easy.
You can buy a new pipe boot and cut off the flat part, then slide the rounded part with the rubber seal attached over the deteriorated seal.
It looks like crap but will work for the DIY’er.